In the restrictions on conforming programs, section 11.1.2.1.2, point 18, it is specified that except where specifically allowed, the consequences are undefined if any symbols in the Common Lisp package are bound as a restart name. However, no such permission is given anywhere, even for the standardized restart names; strictly, this means that the user is unable to participate in the protocols defined by those restarts portably, by binding those names where it makes semantic sense.

read-eval-print-loop, above, is an example from the ANSI standard; it was clearly intended that this should work.
Also, CLHS ABORT description says "Implementors are encouraged to make sure that there is always a restart named abort around any user code so that user code can call abort at any time and expect something reasonable to happen..."

Current practice:

The author knows of no implementation which is incompatible with proposal STANDARDIZED-RESTART-NAMES:ALLOW-BINDING.
CLISP installs an ABORT restart.

Cost to Implementors:

Minimal.

Cost to Users:

None.

Cost of Non-Adoption:

Continued inability to participate in the protocols defined by the ANSI standard.

Benefits:

Enhanced portability of code, and enhanced ability of disparate programs to cooperate in protocols.